Hormonally definately. When our body matures and is ready and able to reproduce. So if our body is ready, our minds will have to be also. Ready for relationships and the drive to have intercourse in order to reproduce offspring and successfully raise it!

Definitely hormones and what is left of whatever natural instinct that we possess. Same goes for playing, most every baby creature on Earth plays as it prepares them for their life later on. If you need more, check out google or wikipedia.

Because like anything else, too much of a good thing isnt good either. There are many diseases and risks that can come with sex especially when immature young people are having sex. Thats the problem and why young people should really not be engaging in sexual behaviours. Its not just physical, there is often baggage that comes with it. Although there is good in sex, there is also bad, just like food. Eating is great too, and taste is also pleasurable however in excess it can be a) unhealthy and b) you will loose appreciation and value for it if u dont respect it!!